Are you sure its broken - not just seized/stuck? or even come off the gear linkage at one end or the other.

IMHO - not necessarily poor technique - but possibly poor maintainence. You have a cable running inside a plastic outer - it gets salt, sand, grime inside it, it corrodes a bit and it gets jammed. If something has broken - probably someone forced it becuase it was already stuck/too-stiff. If you really want to point the finger at someone it should be at whoever didn't look at the cables when people started complaining about them being stiff.

Some other reasons for stiffness - cables too long for the boat, cables routed too sharply/tightly in the boat, adjuster screw on the contol leaver set too tight.

Take the main cowling off the engine and on the side where the cables enter you'll see where they connect to actuator levers. There will be 2 levers, one for the forward/neutral/reverse gear shift and one for the throttle. Disconnect the cables from the levers by pulling out the split pins with some pliers (be careful not to drop any washers etc into the lower engine cowl). When you have done that you have isolated the gear shift lever by the steering wheel from the engine. Now see if you can actuate the levers on the engine by hand. If they move fairly easily then your problem is with the cables or the gear shift lever. Only way to check properly whether its the cables or not, other than looking for visible signs of cracking and rust on the cable outer, is to disconnect the cables from the gear shift lever. This is a bit harder but nothing too bad. Just take the shift lever off the console and take the back plate off. Hope that helps.